1. Field of the Invention
The present invention relates to a throttle valve position sensor used as an electronic control device for an internal combustion engine, and in particular, to such a throttle valve position sensor provided with a plurality of switches for detecting different positions of the throttle valve.
2. Description of the Related Art
In an electronically controlled internal combustion engine for a vehicle, usually a throttle valve position sensor is used for detecting different positions of the throttle valve as a parameter of the engine operating condition. For this purpose, a conventional throttle valve position sensor is provided with a plurality of pairs of contacts (switches) which are made ON or OFF in accordance with different angular positions of the throttle valve, whereby it is possible to determine whether or not the throttle valve is actually in a designated position.
Usually, the computer controlled electronic engine control system is provided with a fail-safe function which determines whether or not the engine is operating properly, and warns the driver of any malfunction. The fail-safe system for the throttle position sensor is usually a means provided for determining whether a connector coupling the throttle position sensor to a microcomputer as a control circuit has been accidentally detached. Further, for the detection by the throttle position sensor of an idle position of a throttle valve of an internal combustion engine, while the engine is operating, a fail-safe system has been proposed wherein a counter is incremented every time the engine is started, and is cleared every time a signal from the idle switch is received. (see Japanese Unexamined Pat. Publication No. 63-28853). It is considered that, during a normal running of the engine, the throttle valve will be in the idling position at least once, and therefore, a zero value will be obtained at the counter when the engine is started, if the idle switch is operating properly. Accordingly, it is determined that a malfunction exists when the value of the counter is larger than a predetermined value. Namely, when the connecter to the idle switch is detached, a signal is not received from the idle switch, and accordingly, the counter is not cleared, whereby it is determined that a malfunction has occurred.
This prior art suffers from a drawback in that a certain amount of time must pass before the malfunction (detachment of the connector) is detected, since the present number of the counter cannot be obtained until after a repetition of a certain number of starting and stopping cycles of the engine, after the detachment of the connector. This problem becomes more serious when the prior art fail-safe technique is applied to the detection of the position of a switch denoting a position of the throttle valve, such as a fully-open position, which has a greater degree of opening than the idling position. This fully-open position switch is not always operated during a period from a starting of the engine to a stopping of the engine, and therefore, the counter maintains a cleared condition, and thus the detection of a malfunction is not possible.